1. Field of the Invention
The invention relates to high performance optical beam steering apparatus of the type having a scanning mirror which is caused to pivot about an axis by a signal-controlled driver.
2. Description Relative to the Prior Art
Mirror-type beam steerers are well known, for example, in information storage and retrieval apparatus; and various mounting and drive arrangements have been proposed for achieving desired steerer performance characteristics. As is discussed in some detail in U.S. Pat. No. 3,758,199, two important characteristics of beam steerers are speed (bandwidth) and capacity (number of resolvable positions). A parameter which is often used to provide an overall measure of performance is the product of speed and capacity.
One known way to improve the performance of beam steerers is to reduce the inertia of the load (mainly the mirror and mounting) which must be driven, and thereby increase the maximum mirror deflection speed. For example, the thickness (and, hence, the mass) of the mirror may be reduced, but there are practical limits to this approach if the structural integrity of the mirror is to be maintained. The size of the mirror can also be lessened to reduce mirror mass, but a reduction in the reflecting surface area has the undesirable effect of reducing the number of spots which are resolvable. This is because of diffraction effects which limit the resolving power of the scanner system as, for example, is discussed at page 55 of Laser Applications, by Monte Ross, Academic Press, Inc., 1974.
U.S. Pat. No. 3,544,201 to Fowler et al describes an arrangement wherein a mirror is arranged close to the effective rotational axis of a bender-type driver, i.e. a driver which flexes along a longitudinal axis in response to an applied signal, an example being a bimorph driver (see U.S. Pat. No. 3,219,850). By concentrating the load mass close to the rotational axis, inertial resistance to rotation is reduced as, for example, compared to a steerer with the mirror mounted perpendicularly at the tip of the bender.
U.S. Pat. No. 3,479,107 to Blythe et al describes a mounting arrangement utilizing two pivots aligned at opposite edges of a mirror. The pivot axis consequently passes through the mirror itself. A frame is placed around the mirror, apparently to provide support to avoid direct connection of the pivots to the mirror. While being suited to heavy-duty applications, such an arrangement introduces considerable mass as a result of the frame. Moreover, the described pivots would prove difficult to fabricate in small sizes. U.S. Pat. No. 3,166,683 describes a steerer apparatus having two pairs of bender-type drivers, each such pair being in an X-configuration. V-shaped brackets mounted to the tips of the bender pairs are arranged with their vertices at the effective pivot axis of the drivers. By mounting the steerer mirror to a shaft which is connected between such vertices, the mirror is closely aligned with the pivot axis and, consequently, a low level of inertial resistance to rotation may be achieved.
U.S. Pat. application Ser. No. 508,775, filed Sept. 23, 1974, now U.S. Pat. No. 3,981,566, describes a hinged lever-action mirror mounting which, in cooperation with a bender-type driver, affords wide angular ranges for mirror deflection. By arranging a fulcrum hinge of such mountings close to a principal axis of the mirror, a low rotational inertia is achieved.
While, as indicated above, there has been considerable development effort directed toward improving beam steerer performance, the demand for increasingly higher capacity information storage and retrieval devices makes even further improvements highly desirable.